1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus employing nuclear magnetic resonance (NMR) phenomena and, more particularly, to a magnetic resonance imaging apparatus with an improved radio-frequency shielding body.
2. Description of the Related Art
The nuclear magnetic resonance is a phenomenon in which atomic nuclei placed in magnetic fields absorb electromagnetic energy at specific frequencies and then emit the energy as electromagnetic waves. A diagnostic apparatus employing the phenomenon senses electromagnetic waves emitted by the atomic nuclei, protons in particular, and processes received signals to obtain diagnostic information of a subject under examination, such as a tomographic image, which contains the atomic nucleus density (the proton density in particular), the longitudinal spin-lattice relaxation time T1, the transversal spin-lattice relaxation time T2, flow, chemical shifts and so on.
The magnetic resonance imaging apparatus for obtaining cross-sectional NMR images of a subject under examination is provided with gradient magnetic field forming coils for forming gradient magnetic fields which serve to obtain position information of a body portion in which magnetic resonance signals are induced and a radio-frequency coil responsive to application of a radio-frequency pulse thereto for radiating a radio-frequency magnetic field serving to induce the magnetic resonance signals in the body portion and detecting the induced magnetic resonance signals. Between the gradient magnetic field forming coils and the radio-frequency coil is disposed a radio-frequency shielding body for interrupting electromagnetic coupling between the gradient magnetic field forming coils and the radio-frequency coil due to the radio-frequency pulse applied to the radio-frequency coil.
The radio-frequency shielding body is generally formed of metallic foil made of a good conductor such as copper. However, eddy currents will be induced in the surface of the radio-frequency shielding body by time-varying gradient magnetic fields formed by the gradient magnetic field forming coils. A problem arises due to the eddy currents in that the rising and falling characteristics of the gradient magnetic fields are deteriorated. As a result, resulting cross-sectional NMR images will have poor quality.
In order to solve the problem with the radio-frequency shielding body, an approach has been proposed in Japanese Unexamined Patent Publication No. 60-177249 according to which metallic foil consisting of first and second conductive regions separated by a relatively narrow nonconductive region, such as a slit, is used to form the radio-frequency shielding body.
Other techniques have been proposed in Japanese Unexamined Patent Publication No. 62-334. That is to say, a metal cylinder, which forms a radio-frequency shielding body, is provided with a lengthwise slit so as to decrease eddy currents by confining them locally or a metal cylinder is formed of plural pieces of metallic foil connected by insulating materials or dielectric materials so as to reduce impedance of a radio-frequency shielding body to a radio-frequency pulse and to thereby reduce undesirable electromagnetic coupling between the gradient magnetic field coils and the radio-frequency coil.
However, such a radio-frequency shielding body is complex in structure and costly because of the provision of the metallic foil with a slit or interposition of a insulating material or dielectric material between pieces of metallic foil.
According to still another proposal in Japanese Unexamined Patent Publication No. 63-290554, the radio-frequency shielding body is formed of a metal cylinder made of copper foil having a thickens not less than the skin depth. The skin depth is the depth in the direction to the center of the cylinder at which the amplitude of electromagnetic waves decays to 1/e (e=2.718 . . , the base of the natural logarithm). More specifically, when electromagnetic waves with the Larmor frequency of protons penetrate into a cylinder made of some metal by a depth of .delta. in the direction to the center of the cylinder, if the amplitude of all the electromagnetic waves decays to 1/e at the depth .delta., then the skin depth is defined as .delta..
In the case of a radio-frequency shielding body with a thickness less than the skin depth, the time constant of eddy currents can be made small so that the time it takes for eddy currents to dissipate is effectively shortened. However, the technique disclosed in the Publication, which is directed to a radio-frequency shielding body made of a metal cylinder, implies that it is impossible to make a radio-frequency shielding body having a thickness less than the skin depth.
As described above, the conventional radio-frequency shielding bodies are complex in structure and costly.